This invention relates generally to gas turbine engines and more particularly to elimination of unfavorable outflow margins in turbine exhaust cases.
For particular operations, particularly in military operations, it is desirable to have aircraft with conventional take-off and landing (CTOL) capabilities, and short take-off vertical landing (STOVL) capabilities. CTOL requires conventional thrusting of the aircraft in the horizontal direction, while STOVL requires thrusting of the aircraft in vertical and intermediate directions. Some dual capability aircraft designs thus employ variable direction exhaust ducts for directing thrust produced by the exhaust nozzle in both the horizontal and vertical directions. Variable direction exhaust ducts typically comprise multiple co-axial exhaust duct segments having angled junctions, whereby the segments can be rotated with respect to each other to redirect the direction of thrust.
In STOVL applications, the lift-fan and roll-posts and the exhaust nozzle work in unison to develop vertical thrust in a powered-lift mode during short take-off/landing segment of the flight (STOVL-PL). The internal engine modules that distribute the flow inside the engine are driven by a dual-spool configuration with high and low-pressure turbines. After the low pressure turbine, the turbine exhaust case (TEC) is one of the last modules in the engine and functions to condition the gas flow before exiting through the exhaust, in either power-lift (STOVL-PL) or during augmentation at up-and-away (STOVL-UAA) flight modes.
In current designs, TEC panels have a local outer region which lacks sufficient internal static pressure for the air flow to discharge into the external gas path via film cooling holes that characterize local TEC design porosity. In turn, insufficient internal pressure leads to a negative outflow-margin in a critical region towards the TEC outer diameter (OD) clockwise (CW) side of the panel. Externally, the gas flow approaches the TEC airfoil at an angle that, upon impact, creates a “bow wave” that surrounds a region that covers the CW side of the vane with high external pressure levels. Simultaneously, if there are decrements to the internal pressure levels, the internal-to-external pressure difference decreases leading to an inflow condition or negative outflow margin.
As the internal cooling flow passes through fan duct blocker in the supply duct before turning into the TEC, a series of pressure drops occurs due to action of roll-post and fan duct blocker flow area variation, particularly during powered-lift. This decreases TEC internal pressure leading to inflow conditions at the TEC critical areas, as is known in the art.
Thermally, and as a consequence of negative outflow margin, the TEC metal temperatures increase to a point closely related to the material limit range of 645-705 degrees Celsius, depending on mechanical stress. This can lead to damage or failure of the TEC panel and airfoils, thus resulting in costly repair or replacement of the TEC panel. Thus, a better system for cooling the critical areas of TEC panels is desirable.